1. Field of the Invention
The present invention relates to a quartz crystal resonator pressure transducer assembly suitable for use downhole in oil, gas, geothermal and other wells, at the wellhead, in industrial applications, for portable calibration devices and in laboratory applications. More specifically by way of example and not limitation, the invention relates to a piezoelectrically-driven temperature-compensated quartz crystal resonator pressure transducer assembly.
2. State of the Art
The general type of quartz crystal pressure transducer assembly as disclosed herein includes a first pressure sensitive quartz crystal resonator, a second temperature sensitive quartz crystal resonator, a third reference frequency quartz crystal resonator, and supporting electronics. For convenience, the terms "crystal" and "resonator" may be used interchangeably herein in referencing a resonating quartz crystal element. The first crystal changes frequency in response to changes in applied external pressure and temperature, while the output frequency of the second crystal is used to temperature compensate temperature-induced frequency excursions in the first and third crystals. The third crystal generates a reference signal, which is only slightly temperature dependent, against or relative to which the temperature and pressure-induced frequency changes in the first crystal and the temperature-induced frequency changes of the second crystal can be compared. Means for comparison as known in the art include frequency mixing or using the reference frequency to count the signals from the other two crystals. The first resonator is exposed via a fluid interface to the external pressure sought to be measured, and all three resonators are thermally coupled to the fluid to provide a rapid thermal response time. The transducer (crystals plus electronics) is calibrated as a complete unit over the intended pressure and temperature range so that all temperature and pressure related effects can be compensated for in the resulting calibration curve-fig coefficients. Exemplary patents for transducers using three crystal resonators, each assigned a function as described above, are U.S. Pat. No. 3,355,949 to Elwood, el al, and U.S. Pat. No. 4,802,370 to EerNisse, et al.
In the Elwood patent, the temperature crystal is used to provide a temperature readout and to compensate for temperature induced frequency changes in the pressure crystal. However, Elwood did not realize that pressure sensitivity of the pressure crystal is a function of the temperature of the crystal. Moreover, all three crystals in Elwood are disclosed as being relatively temperature sensitive, an attribute which makes it more difficult to compensate for temperature dependency of the pressure crystal. Finally, Elwood had no appreciation for the need to have the crystals free, or at least substantially free, of frequency anomalies or activity dips over the intended temperature range of the transducer and the need to have the three crystals have substantially no change in resistance with temperature. Activity dips can cause apparent pressure errors and resistance changes can cause the electronics to cease operation, to operate incorrectly, or require high drive levels.
The aforementioned EerNisse '370 patent isolates a temperature and a reference crystal from the applied external pressure, but all three crystals are temperature-sensitive, and EerNisse does not specifically define desired crystal cuts, (although the temperature resonator is specified as a torsional tuning fork resonator), nor did he specify required or preferred individual crystal performance specifications. The emphasis of the '370 patent is on mounting all three of the crystals in the pressure-transmitting fluid, and matching the heat transfer or conductivity characteristics, and thus the temperature response times, of the temperature and reference crystals to that of the pressure crystal to substantially eliminate temperature gradients produced either by external heating or by pressure-volume heating.
U.S. Pat. No. 4,660,420 to EerNisse recognizes the desirability of selecting a pressure crystal with a crystal cut having substantial independence from temperature-induced frequency changes over the intended range of temperatures, as well as a relatively large scale factor, i.e., greater frequency sensitivity to pressure changes in the range to be measured. For the pressure and temperature ranges experienced in oil and gas wells, an AT-cut quartz crystal is disclosed in EerNisse '420 to possess these attributes.
Another EerNisse patent, U.S. Pat. No. 4,550,610, attempted to select a crystal cut for a pressure crystal which minimized temperature effects, and recommended an SC-cut. However, it was subsequently discovered and disclosed in EerNisse '420 that the SC-cut impeded the ability of the transducer to respond to pressure stresses applied to the resonator housing.
Yet another EerNisse patent, U.S. Pat. No. 4,754,646, discloses the use of an integral housing and resonator section preferably formed from AT-cut, BT-cut, SC-cut or rotated X-cut quartz, but does not distinguish the performance characteristics of any of the various cuts, or recommend a particular cut. Rather, EerNisse '646 seeks to reduce resonator resistance and pressure hysteresis via particular physical configurations of the resonator and its area of joinder to the outer cylindrical shell.
U.S. Pat. No. 3,561,832 to Karrer, assigned to Hewlett-Packard Company, discloses the use of AT-cut and BT-cut quartz crystal thickness-shear mode resonators as pressure and reference crystals, but no temperature compensation is disclosed. In fact, the preferred methodology of the Karrer patent is to maintain the resonators at a constant temperature.
U.S. Pat. No. 3,617,780 to Benjaminson, also assigned to Hewlett-Packard Company, discloses AT-cut and BT-cut resonators for pressure transducers.
It is known to the inventors that certain third party suppliers, such as Clark Oilfield Measurement, Inc., of Tulsa, Okla., modify Hewlett-Packard pressure transducers by the addition of a temperature compensating device, commonly an RTD.
While prior art devices have attempted to address various deficiencies in individual elements of quartz resonator transducer design, those skilled in the art have failed to recognize that the overall design can be enhanced in a synergistic fashion through a judicious selection of quartz crystal characteristics for combined use in the transducer.